Chronotype and mental health: timing seems to matter, but how, why, and for whom?
Authors: Brant P. Hasler
Journal: World Psychiatry (2023)

Despite explosive progress over recent decades in understanding the molecular basis of circadian rhythms and their pervasive role throughout the brain and body, our understanding of a related construct – chronotype – remains incomplete. Historical wisdom going back to Aristotle espouses the benefits of an early sleep-wake schedule for health, financial success, and intellectual prowess. Accumulating research aligns with such relationships, particularly in the realm of mental health: individuals reporting tendencies towards earlier sleep-wake timing also tend to report relatively better mental health and well-being. Nonetheless, questions remain about the nature of chronotype and its relationship to mental health. Here I briefly review the construct of chronotype, note measurement issues that require consideration, discuss evidence for chronotype's relevance to mental health and possible underlying mechanisms, and list potential future directions for research. Clearly defining chronotype is important. Inconsistency in definition has contributed to challenges in comparing and consolidating the ever-expanding literature. Scientific literature has not converged on a definition, but generally conceptualizes chronotype as the tendency for relatively earlier or later sleep and alertness/activity within the 24-hour day, with phenotypes ranging from extreme early to extreme late. Furthermore, chronotype is often conceptualized to index overall circadian timing, and indeed tends to correlate with physiological measures of the central circadian clock, although it appears to be influenced by other non-circadian factors as well, such as homeostatic sleep propensity1. Two approaches to measuring chronotype predominate; one based on preference and one based on actual behavior. The more long-standing approach – morningness vs. eveningness – assesses self-reported preference for the relative timing of sleep and activity (one's own "feeling best" rhythm), producing a score that can be used continuously or to categorize individuals into putatively discrete categories. This approach has long ties to personality literature, and more often treats chronotype as a trait-like psychological construct. The more recent approach, based on the Munich Chronotype Questionnaire (MCTQ), assesses chronotype based on self-reported sleep-wake behavior on "free" days, producing a time that can be ostensibly interpreted as the phase (or timing) of entrainment of the circadian clock relative to the 24-hour light-dark cycle. Although measures from each approach tend to correlate with one another, their conceptual and methodological differences are worth consideration, and have important implications for interpreting their observed relationships to mental health2. Treatment studies demonstrating systematic changes in circadian preference suggest a state-like aspect3, but longitudinal studies that address this question remain scant. Next, factor analyses of preference-type measures suggest the presence of 2+ factors, typically including both a "morning affect" factor that captures how one feels or functions upon rising after sleep (irrespective of when sleep occurred) alongside a factor capturing the relative timing of sleep and/or activity4. This raises concerns that observed associations between chronotype and mental health may be partly driven by how individuals feel upon rising from sleep, rather than by timing per se. The MCTQ's focus on "actual" sleep behavior may obviate this issue. Finally, historical social mores may bias respondents to representing themselves as more morning-type, consistent with my anecdotal clinical experience with Delayed Sleep-Wake Phase Disorder patients who nonetheless steadfastly endorse preferring an early schedule because of perceived advantages for relationships, work and health. Measurement issues aside, the extant literature supports an association between chronotype and mental health. Greater eveningness is consistently associated with a range of mental health outcomes, including anxiety, mood disorders, obsessive-compulsive symptoms, attention-deficit/hyperactivity disorder, schizophrenia and substance use, suggesting a transdiagnostic relationship. Eveningness is also linked to worse physical health, such as obesity and cardiometabolic risk. The most consistent findings are with respect to depression, buttressed by two meta-analyses, and substance use. The meta-analyses5 document substantial heterogeneity across studies as well as generally small effect sizes, although the reliability of the findings despite widely-varying measures of circadian preference is notable. A major limitation of the extant chronotype-mental health literature is its reliance on observational and cross-sectional designs, precluding a determination of directionality. A few studies have documented changes in circadian preference in response to treatment and/or as a predictor of treatment outcomes, suggesting a potential causal role in mental health improvement6. These studies have generally found that shifts towards morningness during treatment parallel improvement in other domains. However, such findings deserve cautious interpretation, as they tend to be small changes on preference-type measures, potentially reflecting people feeling better upon awakening rather than a true change in timing tendencies. The diversity of problems associated with later chronotype raises the possibility of multiple mechanistic pathways, consistent with our growing understanding of the pervasiveness of circadian influence on processes throughout the brain and periphery. A sizeable literature has focused on the most intuitive mechanism – that later chronotype leads to sleep/circadian disruption due to a mismatch with imposed school/work schedules (i.e., circadian misalignment or social jet lag), leading in turn to mental health problems. However, findings remain mixed, and multiple studies have found that chronotype correlates with mental health outcomes beyond any apparent effect of sleep/circadian disruption7. Other mechanistic pathways are plausible and not mutually exclusive from sleep/circadian disruption. Our recent work found substantial associations between eveningness and state-level impulsivity across multiple subdimensions, but the relationships between diary-based sleep timing and impulsivity were weak or absent4. This again raises questions about what aspects of chronotype are most relevant to psychological function. In that study, factor analyses confirmed that the chronotype-impulsivity associations were not driven by the so-called "morning affect" factor vs. the "timing" factor. However, that does not preclude the possibility that there is some trait-like aspect of chronotype that "travels together" with other processes such as impulsivity or sensation-seeking. Indeed, emerging research suggests shared genetic variance between chronotype and risk for mental problems or cannabis use8. Importantly, the nature of chronotype-mental health associations may vary substantively based on age, sex, gender identity, and race/ethnicity, as illustrated by a small but growing literature9. Despite evidence for an association between chronotype and mental health, the nature of this association remains unclear, and leveraging chronotype as a means to inform prevention and/or intervention remains elusive. So, what are the important next steps? These include disentangling "morning affect" vs. sleep/wake timing effects when using preference-type measures; conducting longitudinal studies to elucidate state vs. trait aspects of chronotype and test hypotheses around directionality; and examining the moderating effects of demographics. Improving clarity and consistency in terminology and methodology would help, as the myriad of preference-type measures and scoring approaches challenges comparison across studies. Although MCTQ-based chronotype may be a purer measure of timing than the preference-type measures, it remains subject to biases, including how individuals conceptualize supposed "free" days, and there may be advantages to incorporating objective determination of chronotype via ambulatory measures such as wrist actigraphy and/or physiological measures of circadian phase. Besides reducing self-report bias, including complementary objective measures may help identify which aspects of chronotype are most relevant to mental health: morningness-eveningness preference, patterns of sleep-wake behavior, and/or the underlying circadian clock. Most importantly, attempts to experimentally manipulate chronotype and influence mental health outcomes will be critical to elucidating any causal role of chronotype in mental health. Rigorous attention to these conceptual and methodological issues should greatly facilitate progress on understanding the mechanisms linking chronotype and mental health, thereby enhancing attempts to translate such knowledge into effective approaches to prevention and/or intervention.